RF signal amplifier unit, RF signal transmission device and RF signal transmitting terminal-antenna

ABSTRACT

An RF signals amplification block receives a first RF signal, and comprises means of amplification of this first RF signal so as to deliver a second, amplified, RF signal. This signal must be amplified by a predetermined nominal power so as to be delivered at the output of the block. The block comprises means of measurement of the power of the second, amplified, RF signal, means of coupling the amplified RF signal to a first output of the block, a control signal generated by a central processing unit for controlling the coupling means. The invention also relates to an RF signals transmission device intended to be arranged at the focus of RF signals focusing means comprising a block according to the invention and an RF signals transmission antenna/terminal. Particular application to the tracking of nonsynchronous satellites.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/FR01/02197, filed Jul. 9, 2001, which waspublished in accordance with PCT Article 21(2) on Jan. 31, 2002 inFrench and which claims the benefit of French patent application No.00/09571 filed Jul. 21, 2000.

BACKGROUND OF THE INVENTION

The present invention pertains to the field of telecommunications andrelates more especially to an RF or radiofrequency signals amplificationblock.

1. Field of the Invention

The invention also relates to an RF signals transmission device intendedto be arranged at the focus of RF signals focusing means, in particularbut not exclusively of the type of a parabola or of a lens of Lunebergtype.

2. Related Art

The invention also relates to an antenna/terminal comprising RF signalsfocusing means at the focus of which is arranged at least one device ascited hereinabove. It relates more especially but not exclusively tosuch a terminal comprising two devices as cited hereinabove for thetracking of targets, in particular of the nonsynchronous satellitestype, just one of these two devices being intended to be active at agiven instant.

Hitherto, commercial telecommunications via satellite have been achievedalmost entirely via geostationary satellites, which are especiallybeneficial by virtue of their unchanging relative positions in the sky.However, the geostationary satellite exhibits major drawbacks such asconsiderable attenuations of the signals transmitted related to thedistance separating the user antennas from the geostationary satellite(of the order of 36,000 kilometers, the corresponding losses then risingto around 205 dB in the Ku band) and transmission lags (typically of theorder of 250 ms to 280 ms) thus becoming clearly perceivable andperturbing especially for real-time applications such as telephony,video conferencing, etc. Furthermore, the geostationary orbit, situatedin the equatorial plane, poses a visibility problem in respect of theregions at high latitudes, the angles of elevation becoming very smallfor the regions close to the poles.

The alternatives to employing geostationary satellites are:

-   -   the use of satellites in inclined elliptical orbits, the        satellite then being almost stationary above the region situated        at the latitude of its apogee for a duration of possibly up to        several hours,    -   the implementation of constellations of satellites in circular        orbits, in particular in low orbit (“Low Earth Orbit” or LEO) or        in mid-orbit (“Mid Earth Orbit” or MEO), the satellites of the        constellation flying past in turn within visibility of the user        terminal for a duration of from some ten minutes to around one        hour.

In both cases, service cannot be provided permanently by a singlesatellite, continuity of service demanding that several satellites flyover the service area one after another.

An antenna/terminal as defined in the preamble makes it possible toensure constant communication of the terminal in transmission with theconstellation of nonsynchronous satellites.

Such a terminal usually comprises a parabola or a lens of Luneberg typeat the focus of which are arranged two sources usually comprisingradiating elements, of the type of arrays of “patches”, horns or otherradiating elements for transmitting RF signals. These elements areenergized by an amplifier block making it possible to supply the sourcewith an amplified signal. Upstream of this amplifier block is a centralprocessing unit making it possible to manage the handovers between twosuccessive low-orbit satellites in the radiation space. It is certainthat a single source must be active at a given instant, except duringhandovers where the problem of the activation of the hitherto inactivesource arises.

The role of the amplifier block is to amplify the signal enough to beable to be perceptible by the satellite. It must transmit at a nominalpower predetermined by calculation of the satellite/terminal linkbudget. Specifically, if the transmission is carried out below thenominal power, the satellite recovers a low-level signal whereas if thetransmission is performed above the value of the nominal power, thesignal conveys distortions which may jam, at the satellite level, thereception of the information contained in the signal (in particular ifthe signal modulation performed is of the CDMA type).

The initialization of the inactive amplifier block going from theinactive state to the active state is carried out during handover. Inthe course of handover, the amplifier block must amplify to nominalpower, doing so, from the first few milliseconds onwards, at the risk oflosing the first few useful data items sent by the following satellite.Now, immediate or quasi-immediate transmission at nominal power duringhandover is difficult to regulate, due to factors which may cause thegain of the block to vary, such as component spread, variations in powersupply, transmission frequencies, temperature, thermal resistancebetween the junction of a power transistor and the housing containingthe amplifier, etc. If the effect due to spread of certain parameterscan be reduced by factory calibration, there are others which are moredifficult to control such as the junction temperature, makinginstantaneous regulation of the transmission power almost impossiblewith the accuracy required by the specifications of the satellitesystem. Therefore, the amplifier block takes a certain time beforeamplifying to the predetermined nominal power.

The invention aims to remedy the problems cited above.

SUMMARY OF THE INVENTION

For this purpose, the invention is an RF signals amplification block forcooperating with a central processing unit, comprising a first input forreceiving a first RF signal, means of amplification of this first RFsignal so as to deliver a second, amplified, RF signal, means ofmeasurement of the power of the second, amplified, RF signal,

-   -   means of coupling the amplified RF signal to a first output of        the block, a second input for receiving a control signal        generated by the central processing unit so as to control the        means of coupling as a function of the comparison between the        power of the amplified RF signal and a nominal power value.

Thus, the block according to the invention outputs, during handover, apower signal going from a zero power to the predetermined nominal power.

In order for it to be possible to produce a block according to a massproduction characterized by a modest cost requirement, the said blockcomprises two channels for processing the first RF signal, theamplification means comprising first and second amplification meansarranged respectively on the first and the second processing channel soas each to deliver a third, amplified, RF signal of the same power, thesaid coupling means being able to receive at input the said third RFsignals and to deliver on a second output of the first channel linked tothe first output a difference signal corresponding to the difference ofthe said third signals and on a third output of the second channel a sumsignal which corresponds to the sum of the third signals and isequivalent to the said second signal so as to be measured by the saidmeans of measurement, the coupling means being able to permute thesecond and third outputs.

In this way, the invention makes it possible on the one hand to avoidthe use of a per se expensive microwave frequency switch, on the otherhand to use first and second amplification means which share theamplification function in an equal manner by operating in a parallelmanner. Since they are required to amplify to half the power they havethe advantages of being less expensive and easier to manufacture.

In order for the two amplification means to deliver signals of identicalgains and phases so that the additive or differential combination ofthese signals are respectively constructive or destructive, the saidprocessing channels comprise gain attenuation means and first means ofphase-shifting of signals. In this way, the phase of the signal conveyedon the second channel is altered sufficiently to maximize the powerproduced.

According to one embodiment, the said block being required to amplifywith large gain and to a high power level, at least one of the saidfirst and second amplification means is preceded by a preamplifierusually called a “driver”, and additionally, at least one of the firstand second amplification means comprises a solid state power amplifierSSPA.

According to one embodiment, the said coupling means comprise a hybridcoupler. Thus, the very moderate loss of the hybrid coupler does notcompel the overdimensioning of the point of compression of the SSPA.

According to one embodiment, one of the said channels comprises secondmeans of 180° phase-shifting intended to be controlled by the saidcontrol means so as to carry out the said permutation of the delivery ofthe said signals. The said means of 180° phase-shifting are for examplea low power level phase shifter.

According to a variant, the said coupling means comprise a microwavefrequency switch.

The invention also relates to an RF signals transmission device intendedto be arranged at the focus of RF signals focusing means, of the type ofa parabola or of a lens of Luneberg type, characterized in that itcomprises an amplification block according to the invention.

The invention also relates to an RF signals transmissionantenna/terminal comprising RF signals focusing means at the focus ofwhich is arranged at least one RF signals transmission device,characterized in that the said device comprises a device according tothe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention willemerge from the description of the exemplary embodiments which follow,taken by way of nonlimiting examples, with reference to the appendedfigures in which:

FIG. 1 illustrates a first embodiment of an amplification blockaccording to the invention,

FIG. 2 illustrates a second embodiment of an amplification blockaccording to the invention.

To simplify the description, the same references will be used in thelatter figures to designate the elements which fulfil identicalfunctions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Patent applications Ser. No. 9,805,111 AND 9,805,112 which were filed onApr. 23, 1998 in the name of the Applicant describe antenna/terminalsfor the tracking of nonsynchronous satellites along predefinedtrajectories. For example, in application Ser. No. 9,805,111, theantenna/terminal comprises at least first and secondreceiving/transmitting sources which can move along the focusing surfaceof the Luneberg lens along a line of focal points, the first sourceactively tracking a first satellite and the second source remaining onstandby for active tracking of a second satellite. The signaltransmission chain entails from the internal unit up to the externalunit represented by the antenna/terminal a modulation carried out at thelevel of a modem, at least one transposition making it possible totranspose the signal to high frequencies (to Ku-bands for example for alow-orbit satellite constellation system), followed by an amplificationwith a large gain to a high power level.

In FIG. 1, the amplification block 1 comprises a first input E1receiving a signal RFin originating from a transposition block 2 whichhas transposed the input signal to Ku-band for example. The input E1 islinked to a preamplifier 3 whose output signal is itself amplified by anSSPA power amplifier 4. These two amplifiers are embodied in MMIC(“Microwave Monolithic Integrated Circuit”) technology. The output ofthe second amplifier 4 is linked to a first connection terminal 51 of amicrowave frequency breaker 5. A second connection terminal 52 links, bymeans of a coupler 20, the switch to a diode 7 for detection and formeasurement (known per se) of the power of the output signal from theamplifier 4, the diode being linked to an output S4 of the block. Theline linking the terminal 52 to the diode 7 is matched by a loadimpedance 6 of 50Ω. A third connection terminal 53 links the switch toan output S1 of the block 1. This output is linked to a source placedsubstantially at the focus of the antenna/terminal (not represented).

The handover instants are known to a central processing unit 8, forexample a microcontroller 8 (disposed in the internal unit or in theantenna/terminal) by virtue of an ephemeris comprising thecharacteristics necessary for regulating the trajectories of thesatellites of the constellation. Thus, in operational mode, for ahandover at T0, the microcontroller estimates the new power Pout to beemployed for transmitting the signal to the satellite (as a function ofthe new link budget which may be modified by the use of a newtransmission frequency, for example. This information is transmitted inthe signal transmitted by the satellite). Onwards of a certain time t1prior to the instant of handover T0, the central processing unit causesa power signal Pin to be transmitted to the input E1 and sends a controlsignal Sc to an input E2 of the block linked to an input E3 of theswitch so as to bring about the connection of the terminal 51 to theterminal 52. The output power from the amplifier 4 is then measured bythe diode 7. This measurement is sent to the microcontroller whichbrings about the incrementation of the power signal Pin during the risein the rating of this power. This power measurement and incrementationphase defines the block as being in an inactive state during which itcannot deliver any signal on its output S1.

When the power measured at the output of the amplifier 4 is then equalto the power Pout and the moment T0 is reached, the microcontroller 8brings about via the signal Sc the connection of the terminal 51 to theterminal 53, thus linking the output of the amplifier 4 to the outputS1.

The output power from the device 1 is then equal to the nominal valueapplied in order for the antenna to be able to transmit optimally. Inthis configuration, the block delivering a signal of optimal power Poutat its output S1 is in a so-called active state.

FIG. 2 represents another embodiment of the block 1 according to theinvention. A first input E1 thereof is linked to a preamplifier 3, anoutput of which is linked to two parallel channels 9 and 10. Via thechannel 2, the output of the amplifier 3 is linked to an input of a gainattenuator 11, an output of which feeds it to an input of a poweramplifier 12. Via the channel 10, the output of the amplifier 3 islinked to the input of a phase shifter 13 able to phase-shift the signalat its input by 0° or 180°. The signal thus phase-shifted is thendelivered to a phase-shifter 14 whose function is to alter the phase ofthe signal conveyed on this channel with that of the second channel. Anoutput of this phase shifter is linked to an input of an amplifier 15.An output of each of the two amplifiers 12, 15 respectively delivers anamplified signal RF1′, RF1″ to a respective input E4, E5 of a hybridcoupler 16 which on a first S2 of its outputs delivers the difference ofthe signals RF1′ and RF1″ delivered by the two amplifiers and on asecond S3 of the outputs of the coupler 16 delivers the sum of thesesignals RF1′ and RF1″. The latter output S3, stub-matched by a loadimpedance 6 of 50 Ω, is also linked to a diode 7 for measuring the powerof the sum signal leaving S3. This diode 7 delivers a measurement signalto the output S4 linked to a microcontroller 8.

In order for the block to operate satisfactorily, it is necessary forthe two amplifiers to exhibit an identical gain and an identical phaseso that their respective output signals are constructive. This isachieved by virtue of the attenuator 11 and of the phase shifter 14which are factory-adjusted.

The initialization phase of the procedure for slaving the transmissionpower is triggered as soon as the microprocessor receives the order toprepare itself to manage a handover (for example via an indicative wordin the signal received from the satellite and/or by virtue of theephemeris).

As long as the signal at the output S3 does not transmit a predeterminednominal power, the microcontroller increments the input power Pinaccording to means known per se. Once a threshold value of measurementof power of the sum signal is reached, for example: “Pout minus 20 dB”,the microcontroller determines the corresponding input power Pin, and,the gain of the chain being substantially constant, it then determinesthe new input power Pin to be transmitted to the input E1 of the blockso that the amplification chain should transmit at the output of theamplifier 15 a power Pout equal to the nominal power required (therelation between the input power and the output power of the chain beinglinear).

Once the power Pout is reached at the output S3, the microcontrollerwaits for the order signal at the instant T0 of actual handover so as tosend the command to the phase shifter 13 to invert the phase of thesignal travelling through the second channel so as to invert the outputsof the hybrid coupler. In this way, all of the power is then directed tothe output S1 intended for the source.

The block according to the invention thus makes it possible to calibratethe power supplied to the source with no risk of losing useful dataduring handover, guaranteeing that the first few useful data itemstransmitted will be so at the required nominal power.

The use of two SSPA amplifiers instead of just one makes it possible toreduce the output power of each of them by three decibels and theircompression point correspondingly. The cost of the amplification chainof the terminal is thereby decreased and its reliability increased.Moreover, the 180° switching phase-shifter is an element operating atlow power level whose level scarcely influences the amplification chainsince SSPA amplifiers, often in MMIC technology and having severalstages, provide a large gain predominant over the level of thephase-shifter.

Of course, the invention is not limited to the embodiments describedabove. Thus, the block according to the invention can have the role ofamplification for any type of terminal which is required to generate anominal power at a given instant. Likewise, the terminal is not limitedto the tracking of moving targets such as presented in the introductionof the present patent application. The block can for example be disposedin an antenna/terminal for communication with a geostationary satellite,its role being here again to amplify at a given instant an unamplifiedsignal to the required nominal power, allowing the transmission of asignal with no loss of useful data.

1. RF signals amplification block for cooperating with a centralprocessing unit, wherein said block comprises two channels forprocessing the first RF signal, said block comprising: means foramplifying a first RF signal, comprising first and second amplificationmeans arranged respectively on the first and second processing channelso as each to deliver a third, amplified, RF signal of the same power,means for measuring the power of the third, amplified, RF signal, meansfor coupling the third amplified RF signals and being able to deliver ona second output a difference signal corresponding to the difference ofsaid third signals and on a third output a sum signal corresponding tothe sum of the third signals to a first output of the block as todeliver a second, amplified, RF signal or a signal to the means formeasuring, depending of a control signal generated by the centralprocessing unit as a function of the comparison between the power of theamplified RF signal and a nominal power value, the coupling means beingassociated with means to permute the second and third outputs.
 2. Blockaccording to claim 1, wherein said processing channels comprise gainattenuation means and first means of phase-shifting of signals.
 3. Blockaccording to claim 1, wherein at least one of the first and secondamplification means comprises a solid state power amplifier SSPA. 4.Block according to claim 1, wherein the said coupling means comprise ahybrid coupler.
 5. Block according to claim 1, wherein one of saidchannels comprises second means of 180° phase-shifting intended to becontrolled by said control means so as to carry out said permutation ofthe delivery of said signals.